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Please answer i have a test in 11 hours

  1. Apr 16, 2013 #1
    In a parallel circuit, I know that the brightness stays the same even if you add more bulbs because the voltage is the same as the battery's in each bulb. But, if when you add resistors the REQ is lowered, doesn't that mean the current drops? And if the current drops and the voltage stays the same, wouldn't that make the power lower. Since power is what determines brightness, how can the brightness stay the same???
     
  2. jcsd
  3. Apr 16, 2013 #2

    davenn

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    where are you adding the resistors ?
    in series with each bulb ?

    if so, then you will get current limiting through each resistor, and IF the resistors are all the same value
    then you will get the same amount of voltage drop across each resistor.
    So then each globe in series with its resistor will see a lower but the same voltage.
    the resistor value will determine the voltage drop and therefore the brightness of each globe which will of course not be as bright as when supplied with the original voltage

    say the original voltage is 12V and the globe is a 12V rated globe, but the resistor value drops the voltage to 6V, that means the globe is now only getting 6V across it and I would expect it to be ~ 1/2 the brightness

    Dave
     
  4. Apr 16, 2013 #3
    NOOO... I am adding more bulbs in parallel. It is the most basic parallel circuit.
     
  5. Apr 16, 2013 #4

    berkeman

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    You misunderstand Dave's helpful post. He is assuming you are asking about putting each bulb+resistor segment in parallel.
     
  6. Apr 17, 2013 #5

    CWatters

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    Correct.

    No. Adding resistors in parallel does reduce the equivalent resistance of a circuit but it increases the total current from the battery. However the current through each resistor stays the same.

    Same voltage & same current in each bulb (or resistor) = same power in each bulb (or resistor).
     
  7. Apr 17, 2013 #6
    Yes indeed in a parallel circuit current stays the same through each element.

    Although the brightness might decrease in parallel circuit because of the battery being under load but that depends on the rating of the bulbs you have put in the circuit and the battery parameters.
     
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